There has been a recent resurgence in the interest in unmanned aerial vehicles (UAVs) for performing a variety of missions where the use of manned flight vehicles is not deemed appropriate, for whatever reason. Such missions include surveillance, reconnaissance, target acquisition and/or designation, data acquisition, communications datalinking, decoy, jamming, harassment, or one-way supply flights. This interest has focused mainly on UAVs having the archetypical airplane configuration, i.e., a fuselage, wings having horizontally mounted engines for translational flight, and an empennage, as opposed to "rotor-type" UAVs (UAVs having a vertical take-off and landing capability), for several reasons.
First, the design, fabrication, and operation of "winged" UAVs is but an extrapolation of the manned vehicle flight art, and therefore, may be accomplished in a relatively straightforward and cost effective manner. In particular, the aerodynamic characteristics of such UAVs are well documented such that the pilotage (flight operation) of such vehicles, whether by remote communications datalinking of commands to the UAV and/or software programming of an on-board flight computer, is relatively simple.
In addition, the range and speed of such UAVs is generally superior to rotor-type UAVs. Moreover, the weight-carrying capacity of such UAVs is generally greater than rotor-type UAVs such that winged UAVs may carry a larger mission payload and/or a larger fuel supply, thereby increasing the vehicle's mission efficiency. These characteristics make winged UAVs more suitable than rotor-type UAVs for certain mission profiles involving endurance, distance, and load capability. Winged UAVs, however, have deficiencies that severely limit their utility.
More specifically, winged UAVs do not have a fixed spatial point "loiter" capability nor a vertical takeoff/landing (VTOL) capability. For optimal performance of many of the typical mission profiles described hereinabove, it is desirable that the UAV have the capability to maintain a fixed spatial frame of reference with respect to static ground points for extended periods of time, e.g., target acquisition. One skilled in the art will appreciate that the flight characteristics of winged UAVs are such that winged UAVs cannot maintain a fixed spatial frame of reference with respect to static ground points, i.e., loiter. Therefore, mission equipment for winged UAVs must include complex, sensitive, and costly motion-compensating means to suitably perform such mission profiles, i.e., maintenance of a constant viewing azimuth with respect to a static ground point.
Furthermore, some mission profiles are not conducive to the use of winged UAVs, which require an environment suitable for horizontal takeoffs and landings or an elaborate launching mechanism, i.e., such mission profiles are most expeditiously accomplished by UAVs having a VTOL capability. Rotor-type UAVs having a VTOL capability, for example, are ideally suited for real time reconnaissance, surveillance, and data acquisition missions for front line tactical units. In addition, such UAVs may be used to advantage in ship-board environments where space is at a premium.
Rotor-type UAVs are aerodynamically suited for mission profiles requiring a VTOL and/or loiter capability. The rotor(s) of the main rotor assembly of such UAVs may be operated for vertical takeoffs and landings and to effect hovering at a fixed spatial frame of reference with respect to static ground points. Prior art ducted rotor-type UAV designs, however, experience nose-up pitching moments in forward translational flight. To facilitate a more complete understanding of the aerodynamic characteristics of ducted rotor-type UAVs, and in particular, the nose-up pitching phenomenon, reference is made to FIGS. 1A-1E which illustrate the aerodynamic airflow patterns, pressure distributions (in terms of suction pressure), and pitching moments for a ducted rotor-type UAV having a toroidal fuselage of generally hemicylindrical profile.
FIG. 1A illustrates the aerodynamics of a rotor-type UAV in hover flight, i.e., the UAV is stationary with respect to, and a predetermined distance above, the ground plane. To effect hover flight in a rotor-type UAV, only collective pitch is applied to the rotor R, i.e., the blades of each blade set exhibit the same blade pitch angle regardless of individual blade azimuthal orientation. Rotation of the rotor R induces airflow through the rotor blades, which produces the illustrated pressure distribution PD.sub.R across the span of the blades. Each rotor blade has an equivalent pressure distribution PD.sub.R regardless of its azimuthal orientation such that the pressure distribution across the rotor R is symmetrical with respect to the rotational/fuselage axis of the UAV.
Airflow through the rotor R causes air to be drawn across the upper and duct inlet surfaces of the toroidal fuselage F and to flow through the fuselage duct FD due to the RPM of the rotor R (engine power setting), the arcuate shape of the inlet surface, and the diameter of the fuselage duct FD. This airflow generates a resultant pressure distribution PD.sub.F over the upper and duct inlet surfaces of the toroidal fuselage F that is constant for all azimuthal orientations, as illustrated in FIG. 1A, i.e., the toroidal pressure distribution PD.sub.F is symmetrical with respect to the fuselage axis.
The rotor and fuselage pressure distributions PD.sub.R, PD.sub.F cause lifting forces to be exerted on the rotor R and toroidal fuselage F, respectively, that maintain the UAV in a hover at a fixed spatial point with respect to, and at a predetermined distance above, the ground plane. As an examination of FIG. 1A shows, the lift forces generated by the rotor R and the toroidal fuselage F of a rotor-type UAV are additive in hover flight. Moreover, there are no unbalanced pitching moments acting on the UAV due to the symmetry of the rotor and fuselage air pressure distributions PD.sub.R, PD.sub.F.
The aerodynamic effects resulting from the application of cyclic pitch to a UAV in hover flight are illustrated in FIG. 1B wherein the UAV rotor is simultaneously subjected to collective and cyclic pitch. The individual rotor blades exhibit dissimilar blade pitch angles depending upon blade azimuthal orientation and direction of the applied cyclic input (forward, lateral, aft, or combinations inbetween). Rotation of the rotor R under the influence of both collective and cyclic pitch causes airflow through the rotor blades that produces an asymmetric rotor pressure distribution that is dependent upon the direction of the applied cyclic input. For example, for an applied cyclic input in the forward direction, the asymmetric pressure distribution illustrated in FIG. 1B results wherein PD.sub.RFWD represents the pressure distribution of a rotor blade having a 180.degree. azimuthal orientation (forward) and PD.sub.RAFT represents the pressure distribution of a blade having a 0.degree./360.degree. azimuthal orientation (aft).
An examination of FIG. 1B shows that the magnitude of the resultant pressure distribution is a maximum along the azimuthal orientation opposite the direction of the applied cyclic input and a minimum along the azimuthal orientation in the direction of the applied cyclic input, i.e., PD.sub.RAFT is greater than PD.sub.RFWD. Cyclic pitch, therefore, causes an asymmetric rotor pressure distribution with respect to the center of gravity that results in a net rotor pitching moment M.sub.R in the direction of the applied cyclic input. With respect to FIG. 1B and the example described in the preceding paragraph, the net rotor pitching moment M.sub.R is a counterclockwise moment in the forward direction (180.degree. azimuth).
For the UAV subjected to such cyclic and collective pitch inputs, the airflow through the rotor R induces airflow across the upper and arcuate inlet surfaces of the toroidal fuselage F resulting in an asymmetrical velocity distribution. This asymmetrical velocity distribution produces the asymmetrical toroidal fuselage pressure distribution illustrated in FIG. 1B wherein PD.sub.FFWD represents the pressure distribution across the upper and arcuate inlet surfaces of the toroidal fuselage F at the 180.degree.azimuthal orientation and PD.sub.FAFT represents the pressure distribution across the upper and arcuate inlet surfaces of the toroidal fuselage F at the 0.degree./360.degree. azimuthal orientation.
Cyclic pitch, therefore, also causes an asymmetrical toroidal fuselage pressure distribution with respect to the rotational axis of the UAV that results in a net toroidal fuselage pitching moment M.sub.F in the direction of the applied cyclic input (counterclockwise in FIG. 1B), i.e., PD.sub.FAFT is greater than PD.sub.FFWD. The direction of the applied cyclic input may be varied to cause airflow velocity maxima or minima over the upper and arcuate inlet surfaces of the toroidal fuselage F at any desired azimuthal orientation.
The asymmetric pressure distributions generated by the rotor R and the toroidal fuselage F cause lifting forces to be exerted on the rotor R and the toroidal fuselage F. These lifting forces are additive. In this flight mode, however, there are unbalanced pitching moments acting on the UAV. The net rotor and toroidal fuselage pitching moments M.sub.R and M.sub.F act in concert (moments are additive) to generate a system moment M.sub.S as illustrated in FIG. 1B.
A large portion of the system moment M.sub.S results from the net toroidal fuselage pitching moment M.sub.F (about 50%). Changes in cyclic pitch, which produce changes in net rotor pitching moment M.sub.R, produce similar changes in the net toroidal fuselage pitching moment M.sub.F. The significance of this characteristic is important when considering the amount of cyclic pitch needed to counteract the nose-up pitch instability of UAVs in forward translational flight modes.
FIGS. 1C-1E illustrate the aerodynamic effects acting on a UAV in forward translational flight, i.e., the UAV rotor R is simultaneously subjected to collective and cyclic pitch that causes UAV translational motion with respect to the ground plane. The applied cyclic input is assumed to cause forward motion of the UAV, i.e., a velocity vector along the 180.degree. azimuthal orientation or to the left in FIGS. 1C-1E. Such a flight condition causes airflow through the rotor blades that produces an asymmetric rotor pressure distribution that is dependent upon the direction of applied cyclic input as described hereinabove. An examination of FIG. 1E shows that the magnitude of the resultant rotor pressure distribution is maximum along the azimuthal orientation opposite the direction of applied cyclic input (0.degree./360.degree.) and a minimum along the azimuthal orientation in the direction of applied cyclic input (180.degree.), i.e., PD.sub.RAFT is greater than PD.sub.RFWD. Cyclic pitch, therefore, causes an asymmetric rotor pressure distribution with respect to the rotational axis of the UAV that results in a net rotor pitching moment M.sub.R in the direction of the applied cyclic input (counterclockwise or in the forward direction as illustrated in FIG. 1E).
Airflow through the rotor R induces airflow (V.sub.INFWD and V.sub.INAFT) across the upper and arcuate inlet surfaces of the toroidal fuselage F. In the case of a UAV in forward translational flight, however, the resultant velocity of the airflow across these surfaces at the 180.degree. and 0.degree./360.degree. azimuthal orientations (V'.sub.INFWD and V'.sub.INAFT, respectively) is affected by the free stream velocity V.sub.O due to the translational motion of the UAV. The free stream velocity V.sub.O is additive with respect to the velocity V.sub.INFWD and subtractive with respect to the velocity V.sub.INAFT, as illustrated in FIG. 1C. The net effect of such resultant velocities is the production of asymmetric pressure distributions PD.sub.FFWD and PD.sub.FAFT, as illustrated in FIG. 1D.
Since PD.sub.FFWD is greater than PD.sub.FAFT, the asymmetric pressure distributions generated by the airflows across the upper and arcuate inlet surfaces of the toroidal fuselage F result in a net toroidal fuselage pitching moment M.sub.F in the clockwise direction as illustrated in FIG. 1D. Without cyclic input, the magnitude of the net toroidal fuselage pitching moment M.sub.F is greater than the magnitude of the net rotor pitching moment M.sub.R, and accordingly, the system moment M.sub.S has the same rotational sense as the net toroidal fuselage pitching moment M.sub.F. The system moment M.sub.S without cyclic is a nose-up pitching moment (clockwise) in a UAV in forward translational flight modes as illustrated in FIG. 1E.
There are several options which may be utilized to counteract the nose-up pitch tendency of rotor-type UAVs in forward translational flight modes. One possible option is to utilize cyclic pitch to counteract such nose-up pitching tendencies. The utilization of cyclic pitch to counteract the nose-up pitching tendency of UAVs is disclosed in U.S. Pat. No. 5,152,478. Cyclic pitch is applied to an adversely affected UAV in such manner that the net toroidal fuselage pitching moment M.sub.F is effectively canceled by the net rotor pitching moment M.sub.R. This option is based upon the characteristic described hereinabove wherein changes in cyclic pitch produce changes in the net rotor pitching moment M.sub.R and similar changes in the net toroidal fuselage pitching moment M.sub.F.
The system pitching moment M.sub.S acting upon a UAV in forward translational flight modes comprises the net toroidal fuselage pitching moment M.sub.F, which is a nose-up pitching moment, minus the net rotor pitch moment M.sub.R, which is generally a nose-down (negative) pitching moment. Mathematically, this may be expressed as: EQU M.sub.S =M.sub.F +M.sub.R
During forward translational flight, EQU M.sub.F =M.sub.FF +M.sub.CS
where M.sub.FF is the toroidal fuselage pitching moment normally produced during forward flight modes and M.sub.CS is the toroidal fuselage pitching moment created in response to changes in cyclic pitch (generally negative). Thus, the system moment M.sub.S for a UAV in forward translational flight modes may be expressed as: EQU M.sub.S =(M.sub.FF +M.sub.CS)+M.sub.R
Cyclic pitch is applied so as to change M.sub.CS, thereby causing M.sub.F to decrease. The overall effect is that the net nose-up pitching moment, M.sub.FF +M.sub.CS, is counteracted by the nose-down pitching moment, M.sub.R, at a predetermined cyclic pitch setting. At such predetermined cyclic pitch setting, which results in a negative angle of attack for the UAV in forward translational flight modes, the UAV is aerodynamically trimmed for forward flight, i.e., M.sub.S =0.
While the application of cyclic pitch, as disclosed in the '478 patent, represents a viable option to counteract the fuselage-induced nose-up pitching moments experienced by ducted rotor-type UAVs in forward translational flight modes, this option incurs a performance penalty. More specifically, this option increases the power requirements of the UAV in forward translational flight modes. The application of cyclic pitch to counteract the net nose-up pitching moment also results in lost lift, which must be compensated for by increasing the collective input to the rotor assembly.
Another possible option is to design optimize the toroidal fuselage airfoil profile to counteract such nose-up pitching tendencies of the UAV during forward translational flight modes. The utilization of an optimized toroidal fuselage airfoil profile to counteract the nose-up pitching tendency of UAVs is disclosed in U.S. Pat. No. 5,150,857. The outer aerodynamic surface of the toroidal fuselage is design optimized to provide an asymmetrical toroidal fuselage pressure distribution that produces high lift forces during forward translational flight modes. The high lift forces reduce the required lift provided by the rotor assembly, thereby reducing the undesirable nose-up pitching moment. A reduction in required power is effected by the decreased requirement for rotor lift and the reduced need for superimposed cyclic pitch (moment trim).
While the incorporation of a toroidal fuselage having a design optimized outer aerodynamic surface, as disclosed in the '857 patent, represents a viable option to help counteract the fuselage-induced nose-up pitching moments experienced by ducted rotor-type UAVs in forward translational flight, this option incurs a manufacturing penalty and may have an adverse effect on higher speed flight characteristics. More specifically, a toroidal fuselage having a design optimized outer aerodynamic surface is a more complex structure to fabricate than a UAV having a hemicylindrical outer aerodynamic surface, such that the overall UAV system cost is increased. In addition, a UAV embodying such a toroidal fuselage may require more aft cyclic at higher forward flight speeds.
A need exists for rotary-type UAVs for a wide variety of reconnaissance, surveillance, target acquisition, and/or communication missions, especially at the tactical level. Such UAVs should embody a means for counteracting the undesirable nose-up pitching moments experienced by ducted rotary-type UAVs in forward translational flight modes. Such means should minimize cyclic trim pitch requirements and rotor assembly power requirements while concomitantly providing high hover efficiency.